There are three major reasons why Tarsnap pricing is defined in terms of picodollars ($10^-12) per byte rather than dollars per gigabyte:

1. Tarsnap's author is a geek. Applying SI prefixes to non-SI units is a geeky thing to do.
2. If prices were listed in dollars per GB instead of picodollars per byte, it would be harder to avoid the what-is-a-GB confusion (a GB is 10⁹ bytes, but some people don't understand SI prefixes). Picodollars are perfectly clear — nobody is going to think that a picodollar is 2^-40 dollars.
3. Specifying prices in picodollars reinforces the point that if you have very small backups, you can pay very small amounts.

Unlike some people, I don't believe in rounding up to $0.01 — the Tarsnap accounting code keeps track of everything in attodollars ($10^-18) and when it internally converts storage prices from picodollars per month to attodollars per day, it rounds the prices down to benefit the customer.

The daily price per month of 250 picodollars ($250 × 10^-12) / byte-month are:

Days in month	Price in attodollars / byte / day
28	8928571
29	8620689
30	8333333
31	8064516